Currently, there are many applications that require storing a large amount of heat. In the industrial sector, the recovery, storage and re-use of residual heat could play a significant role for an efficient, economical use of energy. In the generation of energy based on conventional conversion techniques (i.e. gas or oil power plants), the storage of heat could be a useful way to improve the efficacy and recovery thereof, as well as to reduce the nominal energy required to adjust to load peaks.
Significant network stability problems have arisen in electricity generation from renewable energy sources, due to the increase in the amount of electricity generated from these sources and the dependence of the electricity generation on the availability of the resource. For example, solar power plants stop operating at night. The integration of heat energy storage into the operation of such a power plant would help to prevent network stability problems and extend the energy supply period, as well as to increase the efficiency in co-generation, which would contribute to a satisfactory commercialization of these technologies. In the case of autonomous solar thermal plants in remote or isolated electric parks, energy storage is a key element for maximizing the capacity factor and ensuring availability and reducing the mismatch between supply and demand. Most current solar heating systems have storage for a few hours to a day's worth of energy collected, by using thermal energy storage materials (TESMs). Thermal energy storage (TES) helps overcome the intermittency of the solar resource for concentrating solar power (CSP) plants (Dincer and Dost, 1996, “A perspective on thermal energy storage systems for solar energy applications,” International Journal of Energy Research, 20(6), pp. 547-557). TES systems contains a thermal storage mass, and can be based on latent, sensible and thermo-chemical energy storage (Sharma, et al., 2009, “Review on thermal energy storage with phase change materials and applications,” Renewable and Sustainable Energy Reviews, 13(2), pp. 318-345; Gil, et al., 2010, “State of the art on high temperature thermal energy storage for power generation. Part 1 Concepts, materials and modellization”, Renewable and Sustainable Energy Reviews, 14(1), pp. 31-55).
Another example of an application in which thermal energy storage is critical is thermal protection for high energy electronic devices.
Despite the interest, there are very few commercially available high-capacity thermal energy storage systems, due to the high investment cost of existing technologies. In storage systems, an essential aspect to achieve significant cost reduction is the development of low-cost materials that meet the energy requirements for storage in power generation plants, have long-term stability, and a sufficient capacity to prevent over dimensioning of the storage unit.
Existing thermal energy storage technologies are typically based on sensible heat storage systems using liquids (i.e. oils, molten salts) or solids (i.e. metals, ceramics, stones, concrete) as a means of storage. Liquids predominate in applications in the temperature range of 150-400° C., while solids predominate in the temperature interval beyond 500-600° C. For these solids, the energy densities range between 1500-3000 kJ/(m3·K) and the investment costs range from 30 g/kWh for concrete to 400 g/kWh for ceramic materials.
Heat storage based on phase-change materials (latent heat technology) shows a high potential for the development of efficient, economical storage systems, especially for applications that use fluids that undergo a constant temperature process, such as wet steam during condensation or evaporation. The main advantage of phase change materials is their capacity to store/release a large amount of heat in a narrow temperature interval during phase changes.
Salts have been identified as potential candidates for the development of efficient, economical latent heat storage systems. The latent energy or heat involved in the melting/crystallization of salts is normally within the interval between 100-1000 kJ/kg (0.2-2 GJ/m3); these values generally increase with the melting temperature of the salts.
Phase-change heat storage technology using salts utilizes significantly decreased system volumes in comparison to sensible heat technologies, typically by a factor of more than ten, which prevents over dimensioning of the heat exchanger. The low thermal conductivities of salts (<1 W/m/K) are a limiting factor in meeting the energy requirements of the intended industrial applications, however.
Different methods for increasing Thermal conductivity of phase-change materials have been proposed and tested, primarily involving the use of paraffin waxes. The earliest proposed solutions were the use of metal charges, such as aluminum or copper additives, metal foams, or fins. It was found that when these additives were used, the charging and discharging times of the storage system significantly decreased. This solution is of questionable commercial viability because the metal charges add significant weight and cost to the storage systems, and in addition increase the risk of corrosion.
Due to their low density, paraffin waxes supported within a porous structure of an activated silica or carbon catalyst have been proposed as alternatives to the use of metal charges.
Another proposed alternative comprises conductivity enhancement techniques based on graphite additives and graphite foams saturated with or in phase-change materials (PCMs), International (PCT) Pat. Appl. WO98/04644, French Pat. Appl. No. 2715719, and U.S. Pat. Nos. 7,316,262 and 6,399,149 all disclose porous structures (metal or carbon foams, carbon fibers) filled with phase-change materials (PCMs) that melt at low temperatures.
Heat energy storage systems for high temperatures have only been developed relatively recently and are primarily based on the use of salts the conductivity of which is enhanced using graphite. Graphite is used primarily because of its high resistance to corrosion and chemical attack, it's very high thermal conductivity, and its low cost.
Although the efficacy of carbon in enhancing the conductivity of salts has been proven, various problems and limitations have been identified for carbon/salt composites to become a real option for storing heat energy.
The main disadvantage is generally related to the volume expansion of salts when they melt and are subsequently subjected to mechanical stress. Thus, improvements in carbon-salt materials will depend on finding salts with a fairly low relative volume expansion as well as carbon structures that allow for the local management of the volume expansion of salts.
The salts that have already been developed for this use are those that undergo melt at constant temperature (pure salts, eutectic mixtures). Consequently, the use thereof is limited to applications with operational fluids that also undergo a constant temperature process, such as wet steam during condensation or evaporation. If they are to be used in applications that require several temperatures, cascades of appropriate salts must be implemented in order to meet the process requirements in terms of the inlet/outlet temperatures of the operational fluid. While such a cascade could be viable, it would come at the expense of the simplicity of the storage system. In this regard, mixtures of salts that undergo melting in an appropriate temperature interval could be a practical alternative, since there would be no segregation of the chemical components of the salt.
The energy density (latent heat) of the salts known in the art ranges between 100 and 360 kJ/kg. Development of a salt that provides a significantly higher energy density might be a way to increase the compactness of the storage systems and, consequently, reduce investment costs.
Most inorganic phase-change materials present sub-cooling. This is a natural random phenomenon that can lead to significant differences between the melting and crystallization temperatures. In heat energy storage applications, sub-cooling is generally a disadvantage, because it entails using different operating temperatures for charging and discharging.
Compared to the use of salts in latent heat storage, the use of phase change materials (PCMs) is very attractive because of their high storage capacity; their charging and discharging heat at a nearly constant temperature; their minimal maintenance requirements; the ability to produce them at a range of temperatures; and the ease of their integration into existing a power plants. See, for example, Abhat, “Performance studies of a finned heat pipe latent thermal energy storage system,” Proc. Mankind's future source of energy; Proceedings of the International Solar Energy Congress, Pergamon Press, Inc., New Delhi, India, pp. 541-546; Zalba, et al., 2003, “Review on thermal energy storage with phase change: materials, heat transfer analysis and applications,” Applied Thermal Engineering, 23(3), pp. 251-283; Al-Jandal and Sayigh, 1994, “Thermal performance characteristics of STC system with Phase Change Storage,” Renewable Energy, 5(1-4), pp. 390-399; Baran and Sari, 2003, “Phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as PCM in a latent heat storage system,” Energy Conversion and Management, 44(20), pp. 3227-3246; Fouda, et al., 1984, “Solar storage systems using salt hydrate latent heat and direct contact heat exchange. I. Characteristics of pilot system operating with sodium sulphate solution,” Solar Energy, 32(1), pp. 57-65; Morrison and Abdel-Khalik, 1978, “Effects of phase-change energy storage on the performance of air-based and liquid-based solar heating systems,” Solar Energy, 20(1), pp. 57-67; Rabin, et al., 1995, “Integrated solar collector storage system based on a salt-hydrate phase-change material,” Solar Energy, 55(6), pp. 435-444; Velraj, et al., 1999, “Heat Transfer Enhancement in a Latent Heat Storage System,” Solar Energy, 65(3), pp. 171-180; Medrano, et al., 2010, “State of the art on high-temperature thermal energy storage for power generation. Part 2—Case studies,” Renewable and Sustainable Energy Reviews, 14(1), pp. 56-72; Jotshi, et al., 1992, “Solar thermal energy storage in phase change materials,” SOLAR '92: American Solar Energy Society (ASES) Annual Conference Cocoa Beach, Fla., pp. 174-179); all of which are hereby incorporated by reference.
PCMs can store heat using solid-solid, solid-liquid, solid-gas and liquid-gas phase change, though only solid-liquid change is used in PCMs for electrical generation and thermal energy storage. Solid-liquid PCMs increase in temperature as they absorb heat, until PCM reaches the phase change temperature (melting temperature). At the phase change, the PCM absorbs large amounts of heat with minimal temperature change, until the material has undergone a phase transition. When the ambient temperature around a liquid material falls, the PCM solidifies, releasing its considerable amount of latent energy. PCMs are widely used in the art because of the high energy storage density associated with the change of phase.
In a latent heat energy storage system, selection of the appropriate PCM is very important. Most systems known in the art use salt hydrates, paraffins, inorganic acids, clathrates, and eutectic mixtures of organic and/or inorganic compounds. A list of common PCMs for various applications has been compiled by Lange (Lane, 1986, Solar Heat Storage: Latent Heat Materials, CRC Press, Inc, Boca Raton, Fla., which is hereby incorporated by reference). In comparison to organic compounds, inorganic compounds in general have the advantages of having higher latent heat per unit volume, being nonflammable, and having lower costs in comparison to organic compounds (see Tyagi, et al., 2011, “Development of phase change materials based microencapsulated technology for buildings: A review,” Renewable and Sustainable Energy Reviews, 15(2), pp. 1373-1391; Agyenim, et al., 2010, “A review of materials, heat transfer and phase change problem formulation for latent heat thermal energy storage systems (LHTESS),” Renewable and Sustainable Energy Reviews, 14(2), pp. 615-628, both of which are hereby incorporated by reference).
There are some problems associated with salt-based PCMs, however. Inorganic PCMs generally have low thermal conductivity (0.1-0.6 W/m3 K), leading to low heat transfer rates and oxidation on exposure to the heat transport medium (air or heat transfer fluids like oils). In order to overcome such problems, heat transfer enhancement techniques, such as use of extended surfaces and dispersion of high conductivity materials, have been identified and applied (see Jegadheeswaran and Pohekar, 2009, “Performance enhancement in latent heat thermal storage system: A review,” Renewable and Sustainable Energy Reviews, 13(9), pp. 2225-2244, which is hereby incorporated by reference). Another technique to overcome the low heat transfer rate is to encapsulate the PCM within a secondary supporting structure, and use of these capsules in a packed/fluidized bed heat exchanger (see Hawlader, and Zhu, 2000, “Preparation and Evaluation of a Novel Solar Storage Material: Microencapsulated Paraffin,” International Journal of Solar Energy, 20(4), pp. 227-238, which is hereby incorporated by reference). These materials are also encapsulated to prevent water evaporation or uptake, but since they have very low heat transfer characteristics, they tend to solidify at the edges of the encapsulating container, preventing effective heat transfer.
Since the progress of latent heat storage systems mainly depends on ensuring a high effective heat transfer rate to allow rapid charging and discharging, the required heat transfer surfaces should be large to maintain a low temperature gradient during these processes. This requirement can be met efficiently through macroencapsulation.
Macroencapsulated PCMs refer to PCMs incorporated into capsules larger than 1 mm (see Li, et al., 2012, “Fabrication and morphological characterization of microencapsulated phase change materials (MicroPCMs) and macrocapsules containing MicroPCMs for thermal energy storage,” Energy, 38(1), pp. 249-254, which is hereby incorporated by reference). PCM macrocapsules are generally made by preformed shells such as tubes, pouches, spheres, panels or other receptacles with the PCM and sealing the preformed shell (see Cabeza, et al., 2011, “Materials used as PCM in thermal energy storage in buildings: A review,” Renewable and Sustainable Energy Reviews, 15(3), pp. 1675-1695, which is hereby incorporated by reference). The most cost-effective containers are plastic bottles (high density and low density polyethylene and polypropylene bottles for low temperature storage), tin-plated metal cans, and mild steel cans (see Regin, et al., 2008, “Heat transfer characteristics of thermal energy storage system using PCM capsules: A review,” Renewable and Sustainable Energy Reviews, 12(9), pp. 2438-2458; Bauer, et al., 2012, “Characterization of Sodium Nitrate as Phase Change Material,” International Journal of Thermophysics, 33(1), pp. 91-104; Farid, et al., 2004, “A review on phase change energy storage: materials and applications,” Energy Conversion and Management, 45(9-10), pp. 1597-1615; Chou, T. P., Chandrasekaran, C, Limmer, S., Nguyen, C, and Cao, G. Z., 2002, “Organic-inorganic sol-gel coating for corrosion protection of stainless steel,” Journal of Materials Science Letters, 21 (3), pp. 251-255, all of which are hereby incorporated by reference).
The encapsulation process tends to be expensive and difficult, however. Therefore, there remains a need for a thermal energy storage material composition that permits effective heat transfer without requiring encapsulation.
Polymers are low-cost materials that have many mechanical properties that might make them appropriate for use as thermal energy storage materials. Vulcanization (the use of sulfur to cross-link polymer chains) of rubber was discovered more than a century and a half ago. Since then, cross-linked polymer compositions based on natural or synthetic rubber have found uses ranging from automotive to medical to printing. Nonetheless, the properties of rubber are not always ideal for the applications to which they are put. For example, to obtain good mechanical characteristics, the compound must be mixed with so called “reinforcing fillers” such as carbon black or silica. Without the reinforcing fillers, the mechanical characteristics of the rubber compound are too weak.
In many cases, despite the drawbacks of rubber, other polymers are also inappropriate for use in a particular application. For example, while thermoplastic polymers require little or no compounding, they lack elastic properties, and in general it is not possible to modify significantly their characteristics by changes in formulation, thus limiting the types of applications for which they are suitable.
Thus, there remains a long-felt need for a formulation that can be used to produce an elastomer for use as a thermal energy storage material that combines the advantageous properties of rubber (low cost, high chemical and heat resistance, ability to be loaded with filler) with the advantageous properties of other polymers such as thermoplastic polymers.